The invention concerns an actively shielded, superconducting magnet system, in particular, for a high-resolution magnetic resonance spectrometer, comprising a substantially cylindrical cryostat with an axial room temperature bore for receiving a sample, a radio-frequency transmitting and detecting system, and a magnet coil which is superconductingly short-circuited during operation and comprises a main coil and a shielding coil which radially surrounds the main coil, wherein the magnet coil is disposed at a low temperature in a region within the cryostat to surround the sample in the room temperature bore and generate a homogeneous, temporally stable magnetic field at the sample location during operation, the magnetic field meeting requirements for obtaining a high-resolution magnetic resonance spectrum and having a magnetic stray field in an outer space which is substantially reduced compared to a non-actively shielded magnet system.
Superconducting NMR spectrometer magnet systems of the instant Assignee disclose means for stabilizing the magnetic field generated in the measuring volume of a high-resolution magnetic resonance spectrometer by a superconductingly short-circuited main coil located in a cryostat, in particular, in a high-resolution NMR spectrometer, the means comprising one or more compensation coils which are dimensioned and disposed such that they are commonly suitable to largely compensate for field drifts of the superconductingly short-circuited main coil in the measuring volume.
High-resolution NMR spectrometers must have magnetic fields with good temporal stability in addition to extremely good magnetic field homogeneity over the sample volume. Towards this end, the superconducting main coil of the magnet is superconductingly short-circuited during operation. The properties of the superconducting short-circuit switch, the quality of the superconducting coil wires and superconducting joints between individual wire sections of the coil must therefore meet stringent requirements. During short-circuit operation, decay times of the superconducting coil current of several 10,000 years must be guaranteed.
Temporary fluctuations of the magnetic field at the sample location may be compensated for using a so-called lock system. Towards this end, the spectrometer measures a separate NMR signal of a lock substance (i.a. deuterium) in a frequency band provided for this purpose, and its frequency is stabilized through a feedback circuit using a small resistive compensation coil (lock coil) in the room temperature bore of the magnet system.
A superconductingly short-circuited magnet coil keeps the magnetic flux through its bore constant, i.e. the superconducting current changes spontaneously in response to action of e.g. an external disturbance field, such that the total flux through the coil does not change. This does not generally mean that the field in the working volume remains absolutely homogeneous and constant, since the local field distribution of a disturbance and of the main magnet coil do not coincide. Prior art has proposed compensation of these deviations through the design of the main coil geometry, additional superconducting coils or through active control measures (U.S. Pat. No. 4,974,113; U.S. Pat. No. 4,788,502; U.S. Pat. No. 5,278,503).
High-resolution NMR superconducting magnets generally use superconducting shim coil sets to initially homogenize the field at the sample location. During operation, the individual coil sets are charged with a correction current and are superconductingly short-circuited. The shim coil sets may also comprise a so-called B0 coil which can generate a sufficiently homogeneous, small additional field at the sample location. It is thereby possible to finely and precisely adjust the field or the proton frequency to a predetermined value without opening the superconducting circuit of the main coil. Moreover, one has known for some time that, within certain limits, a main coil drift can also be compensated for via a short-circuited B0 coil. Towards this end, the B0 coil must be disposed and dimensioned such that the field decay of the main coil induces a counter current in the B0 coil which causes the field at the sample location to remain constant. This method is limited in that the current through the B0 coil must not get excessively large. This may be limited by the wire which is used. In any case, the contribution of the (low homogeneity) B0 coil must remain sufficiently small that the field homogeneity throughout the sample is not impaired. Moreover, in case of a quench, the B0 coil could be overloaded and destroyed through the required inductive coupling between the B0 coil and the main coil. Corresponding protective means must therefore be introduced, which results in additional expense.
The production of superconducting high-field magnets for high-resolution NMR spectrometers (and also ICR spectrometers) has reached a very high level of quality and reliability. However, an occasional, very expensive magnet system may considerably exceed the specified limits for the drift, while otherwise being quite stable. Compensation of the drift via the lock coil or a B0 coil of the shim system rapidly exhausts their above-mentioned limits and the time intervals between readjustment of the overall field (including associated opening of the superconducting main circuit, introduction of current rods, helium loss etc.) become unreasonably short.
For this reason, there is a need for a superconducting magnet system of the above-mentioned type which compensates for drifts, which are approximately one order of magnitude above those maximally specified, and for a long time, thereby preventing inadmissible deterioration of the homogeneity and stability of the magnetic field at the sample location. It should preferably also be possible to correct finished, drifting main coils.
US-B1 2002/101240 discloses mounting one or more superconducting drift compensation coils within the cryostat in a radially outer region at a temperature which is higher than that of the superconducting magnet coil. The superconducting drift compensation coils may be completely or temporarily superconductingly short-circuited or may also be operated permanently using an external power supply.
In accordance with US-B1 2002/101240, a superconducting compensation coil, in particular, of high-temperature superconducting material at a temperature level above that of the main coil, may be disposed radially outside of the main coil, in particular, in a nitrogen tank of the magnet cryostat or in thermal contact with a refrigerator stage of the cryostat in a temperature range between 20K and 100 K. A current flows through the coil to compensate for the decay of the magnet field through the main field drift at the sample location. A larger distance from the sample location facilitates keeping the compensation field sufficiently homogeneous, e.g. using merely an appropriate Helmholtz arrangement.
The use of superconducting wire permits generation of a sufficiently large current.
Advantageous arrangement e.g. in the nitrogen tank ensures that the superconducting magnet coil need not be changed and installations or modifications on the helium tank are not required.
In contrast thereto, US-B2 6,624,732 proposes operation of a slightly drifting magnet coil such that it is not completely superconductingly short-circuited but has a very small resistance. During operation, the full coil current is always supplied from the power supply or, to be more exact, a slightly higher current, which has an appropriate strength such that the voltage drop over the small resistance is just sufficient to compensate for the drift of the magnet coil. In this manner, the full current is introduced into the cryostat and the power supply noise which is transferred into the magnet coil is negligibly small due to the nearly complete short-circuit across the small resistance.
Since NMR high-field magnet coils are extremely expensive and a coil with a drift which cannot be compensated for, is essentially worthless, there is always a need for inexpensive and operationally tolerable solutions to the drift problem, in particular, in the extreme high-field region.
The last-mentioned variant is problematic in that permanent operation of a high-current power supply is required which i.a. also introduces heat into the cryostat. The variant of US-B1 2002/101240 supplies substantially smaller currents and is locally separated from the magnet coil.
The drift compensation mechanism is advantageously mechanically and also thermally connected to the magnet coil i.a. to prevent e.g. disturbances through vibrations of the drifting coil in the field of the magnet coil, thereby, however, optimally utilizing the advantages of an arrangement outside of the magnet coil.